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Another surprise on sea-level rise: A big Greenland glacier is shrinking fast

Antarctica’s ice-sheet losses are measured in meters per year; this portion of Greenland is losing meters per day.

A major glacier in Greenland is melting and crumbling at a surprisingly fast pace, according to a new research paper, and  entered a period of especially rapid change in 2012.

How major? It holds enough water to raise sea levels worldwide by about a foot and a half, all by itself.

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How fast? Since 2002, 95 percent of its ice shelf has broken off into the sea. Its ice velocity, or the rate at which the glacier flows toward the sea, has accelerated threefold. The pace of thinning in its portion underlain by land has doubled.

It’s the second major piece of research in a month to demonstrate that we know less about the world’s ice sheets than we thought, and far more worrisome than the discovery that portions of the Antarctic ice sheet appear to be growing.

The Greenland glacier’s name is Zachariae Isstrom, and together with an adjacent glacier called Nioghalvfjerdsfjorden, it drains about 12 percent of the overall Greenland ice sheet.

Courtesy of NASA/U.S. Geological Survey
A NASA satellite image shows the Zachariae Isstrom and Nioghalvfjerdsfjorden glaciers.

Nioghalvfjerdsfjorden is retreating, too, but more slowly because its movement is uphill in part. Very good news, because the two glaciers together drain a portion of the ice sheet with enough water to raise global sea levels more than 3 feet.

The pace of change in the Greenland ice has been of greatest concern in regard to sea levels not only because of its great volume – equivalent to perhaps 21 feet of sea-level rise – but because the Arctic regions are generally warming more rapidly than the Antarctic. 

And within Greenland – an island so vast and remote that much of its coastline has yet to be mapped by conventional means – the focus has been on changes in the southern regions.

Measuring from sky and sea

For this paper, a team led by researchers at NASA and the University of California at Irvine set out in 2014 to look at Greenland’s other end. In addition to satellite data, going back decades, that define changes in Greenland’s surface features, they considered gravimeter readings, laser and radar sensing, sonar soundings and a variety of seawater measurements, some collected from a 110-foot fishing trawler refitted as the research vessel (and adventure yacht) Cape Race.

Taking all the measurements into account led the team to conclude, according to the university’s announcement, that Zachariae Isstrom “has come unmoored from a stabilizing sill and is crumbling into the North Atlantic Ocean.”

Losing mass at a rate of 5 billion tons per year, glacier Zachariae Isstrom entered a phase of accelerated retreat in 2012, according to findings published in the current issue of Science.

“North Greenland glaciers are changing rapidly,” said lead author Jeremie Mouginot, an associate project scientist in the Department of Earth System Science at the University of California, Irvine. “The shape and dynamics of Zachariae Isstrom have changed dramatically over the last few years. The glacier is now breaking up and calving high volumes of icebergs into the ocean, which will result in rising sea levels for decades to come.”

“Zachariae Isstrom is being hit from above and below,” said senior author Eric Rignot, Chancellor’s Professor of Earth system science at UCI. “The top of the glacier is melting away as a result of decades of steadily increasing air temperatures, while its underside is compromised by currents carrying warmer ocean water, and the glacier is now breaking away into bits and pieces and retreating into deeper ground.”

In a university-produced video about the close-up data collection from the Cape Race, Rignot makes this striking comparison: Losses from the West Antarctic ice sheet are typically measured in meters per year; losses at Zacharaie Isstrom have been measured at rates of meters per day.

(An aside: If your household includes anyone young enough to be considering a new career, the notion of Arctic research from the decks of the Cape Race may have some appeal.  

(Apart from the cool science being done aboard the 294-tonner, the vessel has a variety of comforts, including staterooms large enough for queen-size beds, a sauna, and a mahogany-paneled saloon with woodstove and piano.  And it has retained its main and mizzen masts along with their standing rigging, so I suppose it’s at least possible she could still move under sail.

(But diesel power is of course the preferred mode in the fjords of Greenland; Rignot can be heard on video advising that they’ve entered a region of icebergs large enough that bumping one at seven knots would mean “a call to the insurance company.”)

Maria Stenzel/University of Califorinia, Irvine
Eric Rignot on the deck of the Cape Race

The Mouginot/Rignot paper is fairly dense going, too, for the most part but it is brief and Science is making it freely available here.

Good coverage elsewhere

Of the fairly minimal media coverage the paper has attracted so far, I suppose my favorite piece was Andrea Thompson’s in Scientific American, which brought both clarity and context to the findings. A sample:

In their data, the researchers saw that while Zachariae was stable for decades, that all changed when a huge chunk of its ice shelf broke off in 2002-03, after which the remainder continued to crumble. Today it is only 5 percent of the size it was in 2002.

With that stabilizing force reduced, the glacier steadily retreated until 2012, when it noticeably sped up and started shedding ice from its grounding line, the point where ice transitions from land to the sea. The team calculated that between 2011 and 2015, the grounding line retreated the same amount as it had in the 15 years prior, a quadrupling of its retreat rate.

For a brand-new and broader look at the whole subject of sea levels and the ice at the top of the world I commend Jon Gertner’s “The Secrets in Greenland’s Ice Sheet” in the New York Times Magazine last week. A sample, with paragraphing added:

Earth’s geological history, together with the contours of ancient shorelines, tell us two things about ice-sheet collapse. The first is that great quantities of ice can fall into the ocean rapidly, at rates far exceeding what is happening today. The second is that even if sea-level rises don’t happen quickly in the near future, they will happen eventually. As [Penn State glaciologist Richard] Alley told me, the historical record points in two directions: ‘‘Sea-­level rise could be scary in magnitude but not in rate. Or it could be scary in magnitude and rate if our warming reproduces what happened in the past.’’

We might imagine a rapid collapse of the ice sheets in B-movie terms: sudden and terrifying, with enormous waves of water cresting over beachside homes in Malibu, Calif., and nature reclaiming the Rockaways. In truth, the remoteness of the sources of new icebergs means no devastating tsunamis.

Because the calving would happen over the course of many decades rather than weeks, the catastrophe would manifest over time. First there is water in the basements, gutters and subways; then, storms regularly bringing water into the streets. Year by year, the rise accelerates. Brine infiltrates drinking-water systems and sewer plants; electrical grids spark out. Flood-insurance policies are discontinued, and home values plummet. Row by row, seaside homes are abandoned. Still the rise continues.

Large­scale evacuation then becomes imperative — as long as inland cities and funds are available for relocation. In low­lying countries, however, the implications of significant sea-level rise, and the occasional storm surges that amplify the floodwaters, move beyond the economic to the existential.

‘‘On these longer time scales,’’ says Anders Levermann, a sea­level expert at the Potsdam Institute for Climate Impact Research, ‘‘the magnitude of the sea­level rise could get so big that we have to evacuate New York, Calcutta, Hong Kong, Shanghai, Hamburg and most of the Netherlands.’’